(a) Field of the Invention
The present invention relates generally to lithium ionic batteries and, more particularly, to anode materials for use in rechargeable lithium ionic batteries.
(b) Description of Related Art
As a consequence of the rapid advancements made in microelectronics, the size and weight of portable electronic devices have been significantly reduced. This has spurred the development of many business- and entertainment-related portable electronic devices that require safe, long-life, and high energy density rechargeable batteries.
A battery or voltaic cell generally includes two chemicals or elements with differing electron-attracting capabilities that are immersed in an electrolytic solution and connected to one another through an external circuit. These two chemicals can be referred to as an electrochemical couple. In a zinc-acid cell, for example, the electrochemical couple is a zinc-hydrogen ion couple. The reaction that occurs between an electrochemical couple in a voltaic cell is an oxidation-reduction reaction.
The mechanism by which a battery generates an electric current typically involves the arrangement of chemicals in such a manner that electrons are released from one part of the battery via an oxidation-reduction reaction and made to flow through an external circuit or cell connection to another part of the battery. The element of the battery at which the electrons are released to the aforementioned circuit is called the anode, or the negative electrode. During discharge, oxidation reactions occur at the anode. The element that receives the electrons from the circuit is known as the cathode, or the positive electrode. During discharge, reduction reactions occur at the cathode.
At rest, a voltaic cell exhibits a potential difference (voltage) between its two electrodes that is determined by the amount of chemical energy available when an electron is transferred from one electrode to the other. The current that flows from a cell is determined by the resistance of the total circuit, including that of the cell itself. Further, a voltaic cell has a limited energy content, or capacity, that is generally given in ampere-hours and determined by the quantity of electrons that can be released at the anode and accepted at the cathode. When all of the chemical energy of the cell has been consumed (usually because one of the electrodes has been completely exhausted) the voltage falls to zero and will not recover unless the battery can be recharged. The capacity of the cell is determined by the quantity of active ingredients in the electrode.
There are two major types of voltaic cells, primary batteries and secondary (or "storage") batteries. Primary batteries are constructed so that only a single continuous or intermittent discharge can be obtained. Secondary batteries are constructed so that they can be discharged and then recharged to approximately their original state. Secondary batteries often include several identical voltaic cells.
Of the various types of secondary batteries currently available commercially, the lead-acid type is the most widely used, serving as a power source for the electrical systems of automobiles, for example. The active constituents of a lead-acid battery are sulfuric acid and two sets of plates (electrodes), one containing pure, elemental lead and the other lead dioxide. Each component cell includes several of these plate pairs connected in parallel and is capable of delivering approximately two volts. Therefore, three or six cells are typically connected in series to make a six or twelve volt battery, respectively. During discharge, the plate materials are converted into lead sulphate and the sulfuric acid is depleted. Discharge stops before all component chemicals are exhausted, usually when the acid can no longer physically reach the active materials. Charging the battery by passing a direct current through it reverses the chemical changes described above, displacing the sulphate from the plates and causing a rise in the specific gravity of the sulfuric acid.
Another type of secondary or storage battery is the nickel-cadmium battery, which operates similarly to the lead-acid battery described above, but with different chemical constituents. This type of battery can include a nickel hydroxide cathode and a cadmium anode immersed in an electrolytic solution of potassium hydroxide.
Presently, the most widely used rechargeable batteries are believed to be aqueous solution secondary batteries such as lead-acid, nickel-cadmium and nickel metal-hydride batteries. However, at least partially as a result of their relatively low energy densities, these aqueous solution batteries do not provide sufficiently long battery life.
Prior to approximately 1991, substantial worldwide research and development efforts were dedicated to the development of rechargeable lithium metal-based anode batteries because it appeared such batteries could produce energy densities as high as 150-200 Wh/kg (watt-hours per kilogram). However, it was found that this type of lithium battery could not be produced or manufactured successfully, at least on a commercial scale, because of severe safety problems associated with the metallic lithium anode interface during cycling. Safety problems with such batteries can include the danger of fires and explosions resulting from the lithium metal present therein.
In approximately 1991, Sony Corporation introduced a lithium ion ("Li-ion") battery. The fundamental difference between this battery and the aforementioned lithium metal battery is that the lithium (metal) anode was replaced by an anode made of a carbon material. In this battery, lithium metal need not be present at any time. Rather, lithium ions are dissolved in a non-aqueous electrolytic solution. These lithium ions could be transported back and forth between an intercalation compound cathode (normally containing a lithiated transition metal oxide such as Li.sub.x CoO.sub.2 or LiMn.sub.2 O.sub.4) and a carbon intercalation anode (such as Li.sub.x C.sub.6). In this arrangement, metallic lithium is not plated and stripped in the battery, and the safety of the system was significantly improved. Such batteries are described in Megahed et al., "Rechargeable Nonaqueous Batteries," Interface (Winter 1995); Azuma et al., "Extended Abstracts of Fall Meeting Electrochemical Society of Japan," Oct. 12-13, 1991; von Sacken, "Extended Abstracts, Seventh International Meeting on Lithium Batteries," May 15-20, 1994; and Ozawa et al., "10th International Seminar on Primary and Secondary Battery Technology and Application," March 1993, the respective disclosures of which are hereby incorporated herein in their entirety.
The aforementioned commercial Li-ion battery anode was composed of a non-graphitizable carbon material having a low crystalline property formed by heat-treating an organic material such as a furfural alcohol resin at a relatively low temperature. Non-graphitic carbon materials are generally manufactured with an intermediate temperature heat treatment, for example, about 700.degree. C. to about 1300.degree. C. A Li-ion battery including this non-graphitic carbon anode, a LiCoO.sub.2 cathode, and a liquid electrolyte comprising a propylene carbonate ("PC") solvent has been commercially available since approximately 1991.
Other carbon materials such as graphite, which has a high degree of crystallinity, have been studied as an anode material for Li-ion batteries. Graphite, however, had been considered to be less suitable for the anode because graphite decomposes propylene carbonate, which is the principal solvent used in the aforementioned non-aqueous Li-ion battery. However, it has been found that by replacing PC with ethylene carbonate ("EC") as the principal solvent, the rate of solvent decomposition is small enough to allow for use with natural graphite. Thus, a non-aqueous Li-ion secondary battery which has an anode formed of graphite and a non-aqueous liquid electrolyte using EC as the main solvent has been developed.
Although the solvent decomposition problem of graphite anode batteries has been reduced, the powder grains of graphite typically exhibit a plate-like shape with the shorter dimension along the crystallographic "c" axis (perpendicular to the plates of carbon atoms). This typically gives rise to anisotropic (dependent upon direction) flow properties under shear, so that an anode made of a rolled or pressed foil of graphite powder might exhibit a high degree of preferred orientation, undesirably resulting in particles with a c-axis perpendicular to the electrode surface. Such an orientation has been found to reduce the kinetics of lithium intercalation, since in this case the lithium ions must intercalate into the graphite at the edges of the plate-like structures which are oriented away from the electrolyte-carbon interface.
A graphite anode having a high true density (e.g., greater than 2.1 g/cm.sup.3) and a high intercalation capacity (372 mAh/g) exhibits a high energy density, as well as a relatively flat discharge curve. Therefore, batteries assembled with a graphite anode can provide the advantage of generating little or no energy loss in voltage conversion by an electronic device. However, an anode formed of planar oriented graphite, as described above, can require lithium ions to diffuse longer distances during charge and discharge cycles, and is likely to cause polarization. As a result, if the battery is charged with a relatively high current, an overpotential caused by the slow diffusion rate makes the anode potential more negative than the potential of lithium metal, possibly causing lithium metal to precipitate or plate on the surface of the graphite anode.
Therefore, it would be advantageous to provide an anode material having a relatively high capacity of graphite material in a form that will also have better cycle life than graphite.
Non-graphitic carbon materials typically include numerous disordered areas and some graphite-like crystallites in its particles. The coherence lengths of crystallites (aligned graphite sheets) are in the ranges of about 15-40 angstroms and about 20-100 angstroms for c-axis and a-axis, respectively. Due to the disordered structure, there are many orientations and many diffusion routes, thereby causing the diffusion rate of lithium ions into non-graphitic carbon to be relatively high. The carbon layers of non-graphitic carbons are cross-linked, resulting in a stable structure. A Li-ion battery utilizing a non-graphitic carbon anode shows high current output and less polarization. Even when such a battery is charged at high current densities, with such anodes there is no lithium metal precipitation on the carbon particle surface. Such unique characteristics of non-graphic carbon anode materials make the Li-ion battery advantageous for high charge and discharge currents and excellent rechargeability. These batteries are typically able to endure more than 1000 cycles. However, the lower true density of non-graphitizable carbon (&lt;1.70 g/cm.sup.3), and the lower intercalation capacity of lithium ions into non-graphitic carbon (about 80% of the capacity of graphite) are the major disadvantages of non-graphitic carbon anodes.
To summarize, graphite anodes typically show high energy density and flat voltage discharge characteristics, but poor cycle life and severe capacity decay. Non-graphitic carbon anodes typically have excellent rechargeability and large charge/discharge current capabilities, but low packing density and low energy density.
In order to find a structure that combines the benefits of both graphite and non-graphitic carbons for anode electrode applications, a core-shell structure has been proposed by Isao Kuribayashi et al., "Journal of Power Sources," Vol. 54, pp. 1-5 (1995). The carbonaceous materials disclosed therein have an outer shell made of coke-like carbon and a core composed of graphite or pseudo-graphite. Each powder consists of natural graphite, spherical artificial graphite and pseudo-graphite from heat-treated mesophase-pitch beads, or polyhedral artificial graphite from coal tar, and was coated with pitch-blended phenol resins (modified phenol resins) in a kneader and then heat-treated at up to 1200.degree. C. in a nitrogen gas atmosphere. The distance between graphitic planes or plates in the carbon ("d(002)") of the graphite core and petroleum coke shell were 3.36 and 3.54 angstroms, respectively. The disclosed process utilized mechanical mixing, and the petroleum coke shell from phenol resins was a graphitizable carbon (d(002)=3.54). However, because the mechanically-made coating was not much different from the graphite (as shown by the d spacing), it is understood that the diffusion rate of lithium ions and battery performances could not be significantly improved by means of the disclosed technique.
A method for improving the rechargeability of lithium ion batteries assembled with graphite anodes has also been proposed by Atsuo et al. (European Patent Publication No. 627,777 dated Dec. 7, 1994). In this method, the carbon material for the anode contains both graphite and a non-graphitic carbon material formed of at least one of a non-graphitizable carbon material or a graphitizable carbon material. The graphite has a true density of 2.1 g/cm.sup.3 or greater, an interplanar distance of (002) of less than 0.340 nm, and a c-axis crystallite size of 14 nm or greater. The non-graphitizable carbon material is disclosed as having a true density of 1.70 g/cm.sup.3 or less and an interplanar distance of 0.37 nm or greater. In this disclosure, a graphite powder was formed by grinding natural or artificial graphite. A non-graphitic carbon was produced by grinding and oxidizing petroleum pitch or coal pitch to form a carbon precursor. The carbon precursor was ground and then heat-treated to form a non-graphitizable carbon powder.
The graphite powder and the non-graphitic carbon thus produced were mechanically mixed at various ratios to form concomitant bodies of graphite and non-graphitic carbon material. With the resulting concomitant body used for the anode, a non-aqueous liquid electrolyte secondary Li-ion battery (coin or cylindrical shapes) was produced. By increasing the content of non-graphitic carbon in the whole concomitant body, this document states that polarization values were decreased, and cycling life and battery capacity were increased. However, this composite anode is a mechanical mixture of different graphite powders and different non-graphitic carbons. It is believed that the mechanical mixing process disclosed therein could not ensure the optimum distribution of non-graphitic carbon and graphite powders, particularly on large scales.